Recent advances in the development of solid-state light-emitting devices such as light-emitting diodes (LEDs) including semiconductor LEDs, small molecule organic light-emitting diodes (OLEDs) and polymer light-emitting diodes (PLEDs), have made these devices suitable for use in general illumination applications, including architectural, entertainment, and roadway lighting, for example. As such, LEDs are becoming increasingly competitive with light sources such as incandescent, fluorescent, and high-intensity discharge lamps.
LEDs offer a number of advantages and are generally chosen for their ruggedness, long lifetime, high efficiency, low voltage requirements, and above all the possibility to control the colour and intensity of the emitted light independently. They can provide a significant improvement over delicate gas discharge lamps, incandescent bulbs, and fluorescent lighting systems while being capable of providing lighting impressions similar to these technologies.
When drive current is applied to an LED, Joule heating can result in transient thermal gradients exceeding about 3000° C./cm as shown by Malyutenko et al. in “Heat Transfer Mapping in 3-5 um Planar Light-Emitting Structures,” Journal of Applied Physics 93(11), 2003:9398-9400. In addition, localized peak temperatures as high as about 150° C. can be reached under normal operating conditions as shown by Barton et al. in “Life Tests and Failure Mechanisms of GaN/AlGaN/InGaN Light-Emitting Diodes,” SPIE Vol. 3279, 1998, pp. 17-27. Heat sinking can be used to reduce the average junction temperature of an LED die however this can typically only be done under steady-state conditions since when the drive current is first applied, the localized peak temperature will likely exceed the steady-state value until the generated heat is dissipated through the heat sink.
The thermal stresses due to rapidly heating and cooling of components within lighting systems can lead to a number of failures such as the fracture of wire bonds and lift off of a LED die from the package. As reported in the publication, “Application Brief A04: LED Lamp Thermal Properties,” Agilent Technologies 2001, undue thermal stresses beyond the recommended operational limits can greatly reduce the mean-time-between-failure (MTBF) for LED wire bonds. Also reported in this document is the fact that for temperatures over the range of about 100° C. to 115° C., each increase in maximum storage temperature excursion by about 5° C. lowers the mean number of temperature cycles to failure by about a factor of five. Thus, an LED lamp will fail with about 100 times fewer temperature cycles over a range of about −40° C. to 115° C. than a range of about −40° C. to 100° C. Agilent and other LED manufacturers state that their LEDs can withstand thousands of temperature cycles over a temperature range of about −55° C. to 100° C., however this ability is typically determined under non-operating, or storage, conditions. Assuming that these thermal cycles occur within an environmental chamber with a cycle time of minutes, the thermal gradients and resultant mechanical stresses on the wire bonds are likely to be small, as the LED package will be able to substantially maintain thermal equilibrium, depending on the thermal constant of the heat sink. A worst-case scenario may therefore occur when a LED is connected through a low thermal resistance link to a heat sink with a large thermal constant, and where full drive current is applied to the LED when it is in thermal equilibrium at a low ambient temperature, for example as in the case for an outdoor luminaire operated in winter conditions. For example, if a luminaire is cycled through a sequence that is about ten minutes in length, it is conceivable that this potential worst-case scenario can occur dozens of times in one night.
Thermal stress in lighting systems due to excessive rapid heating and cooling can be managed by controlling the device temperature and the device temperature gradients during operation. For example, U.S. Pat. No. 4,680,536 discloses a dimmer circuit with an input voltage compensated soft start circuit for an incandescent lamp. The dimmer employs a feed-forward phase control mechanism that controls power provided to a load during transient ON/OFF cycles. The invention however, only works with alternating currents which are not suitable for LEDs since they are typically operated with direct currents. U.S. Pat. No. 6,573,674 also discloses a circuit for controlling a load supplied with an alternating current and is similarly unsuitable for LEDs.
In addition, U.S. Pat. No. 4,952,949 describes a form of temperature compensation for an LED print head. The forward voltage of a dummy LED is cyclically measured in order to derive the junction temperatures of an array of LEDs, and subsequently the respective device currents that are necessary to achieve a desired light output are determined. The invention however, does not protect the LEDs from thermal stress resulting from storage at low ambient temperatures for example.
U.S. Pat. No. 5,825,399 also describes thermal compensation for an LED print head for maintaining proper printer calibration as the LED warms up due to thermal energy generation from the drive current. However, thermal stress during the ON/OFF transient periods is not considered.
U.S. Pat. No. 4,633,525 describes a method of thermal stabilization wherein the LED is reverse-biased with a voltage sufficient to induce a current flow equal to the forward-biased current flow, thereby maintaining a constant junction temperature. This method of maintaining the junction temperature of an LED can be inappropriate for some LEDs as the reverse breakdown voltage of the LED may need to be exceeded in order to achieve the desired result.
U.S. Pat. No. 5,262,658 discloses an LED die wherein heater elements are positioned along the sides of the LED. This method of suppressing thermal effects however, results in additional power consumption in order to maintain the temperature of the LEDs at a desired level, which may be relatively large when LEDs are being used in an outdoor environment, for example.
Furthermore, U.S. Pat. No. 5,030,844 discloses a DC power switch for inrush prevention and U.S. Pat. No. 5,309,084 discloses an electronic switch suitable for fading ON/OFF control of electrical equipment like lamps and motors. In both of these disclosures however, the rate at which a signal is provided to a load is predetermined and may not sufficiently reduce thermal stresses in cold environments, for example.
This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.